Vacuum pumping arrangements used to pump fluid from semiconductor tools typically employ, as a backing pump, a multi-stage positive displacement pump employing inter-meshing rotors. The rotors may have the same type of profile in each stage or the profile may change from stage to stage.
A number of semiconductor processes can produce a significant amount of by product material in the form of powder or dust, especially if the process gas is condensable and sublimes on lower temperature surfaces. This material can be formed in the process chamber, in the vacuum line, or “foreline”, between the chamber and the pump, and/or in the vacuum pump itself. The material can be in a soft powder form or it can become hard and compacted. Within the pump, such material can accumulate within the vacant running clearances between the rotor and stator elements in the pump, reducing the size of the clearances. While the pump is running continuously, this does not present any problem, but in the event that the pump is switched off (either intentionally for system maintenance or unintentionally in the event of an unexpected power supply interruption) the pump will cool and the size of the running clearances will decrease. Depending on the state of the powder accumulation, this could cause the accumulated material to become compressed between the rotor and stator elements. Due to the relatively large surface area of potential contact that this creates between the rotor and stator elements, such compression of by-product material can significantly increase the frictional forces opposing rotation. When it is then attempted to re-start the pump, the torque available from the pump motor may be inadequate to overcome these frictional forces, resulting in a re-start failure. The current trend towards inverter driven pumps increases the likelihood of re-start failure, as such motors have a lower starting torque than direct-on-line motors conventionally used to drive the rotor elements of vacuum pumps.
It is an aim of at least the preferred embodiment of the present invention to seek to improve the re-start performance of vacuum pumps.